Temperature guage issues...

I know this has been talked about before, but I"m having an issue with my 97 XJ and hope someone can offer some insight...

I recently replaced the thermostat while doing a flush and fill. I flushed the radiator and heater core out very well. I replaced the 180 thermostat that I had put in it earlier with a 195 thermostat.
Since I did this, my heat is working much better, but now the temperature gauge will suddenly spike and the check gauges light will come on while driving. It will stay on for a minute or so and then drop back down to 215 or so.
It used to do this before and that is why I put the 180 thermostat on it. It did not do this with the 180 on it but my heat did not work very well at all.
The temperature here has been in the 40's the past few weeks so I don't know what effect that may have.

Please help!

Thanks...

The sensor in the thermostat housing is probably shorting internally, or the wire has an open that is touching ground periodically. Easier to replace the sensor. Either grab a couple next time you go to the junkyard so you can test with them, or just buy a new one from the dealer or Standard brand from rockauto (stay away from house brands)

Maybe the sensor, or perhaps the radiator cap is faulty. Loss of cooling system pressurization results in steam pockets.

Something else that can cause this is poor flow through the heater or the heater valve (you don't have one). The Heater keeps a good flow going around the thermostat (it bypasses the thermostat) so you get a quicker reaction from the thermostat and not pulses of too cold and too hot.

Another thing that can cause this is putting the thermostat in bassackwords.